The present disclosure relates generally to activated carbon materials and more specifically to activated carbon materials having low-oxygen content and high-capacitance. The disclosure relates also to high power density energy storage devices having carbon-based electrodes comprising such activated carbon materials.
Energy storage devices such as ultracapacitors may be used in many applications such as where a discrete power pulse is required. Example applications range from cell phones to hybrid vehicles. Energy storage devices typically comprise a porous separator and/or an organic electrolyte sandwiched between a pair of carbon-based electrodes. The energy storage is achieved by separating and storing electrical charge in the electrochemical double layer at the interfaces between the electrolyte and the electrodes. Important characteristics of these devices are the energy density and power density that they can provide, which are both largely determined by the properties of the carbon-based electrodes.
Carbon-based electrodes suitable for incorporation into high energy density devices are known. The carbon materials, which form the basis of such electrodes, can be made from natural or synthetic precursor materials. Known natural precursor materials include coals, nut shells, and biomass, while synthetic precursor materials typically include phenolic resins. With both natural and synthetic precursors, carbon materials can be formed by carbonizing the precursor and then activating the resulting carbon. The activation can comprise physical (e.g., steam) or chemical activation.
A property of the carbon that can influence its success when incorporated into high energy density devices such as electric double layer capacitors (EDLCs) is the material's specific capacitance. Higher specific capacitance generally results in a higher volumetric energy density of the resulting device. In addition to the foregoing, reducing the oxygen-content in the carbon materials can beneficially increase the cycle life of the device. Specifically, during conventional processes for synthesizing activated carbon, oxygen may be incorporated into the carbon in the form of surface functional groups, which can adversely affect the properties of the carbon. Accordingly, it would be an advantage to provide activated carbon materials as well as methods for making activated carbon materials having a high specific capacitance and low-oxygen content. Such materials can be used to form carbon-based electrodes that enable higher energy density devices.
According to one aspect of the disclosure, activated carbon materials that are suitable for incorporation into carbon-based electrodes for use in ultracapacitors and other energy storage devices is derived from natural non-lignocellulosic materials. By using non-lignocellulosic materials as a precursor for the activated carbon material, economically viable, high power, high energy density devices can be formed. As used herein, unless expressly defined otherwise, “natural, non-lignocellulosic carbon precursor” means at least one natural, non-lignocellulosic carbon precursor.
According to a further aspect of the disclosure, a low oxygen content activated carbon material is prepared by heating a natural, non-lignocellulosic carbon precursor in an inert or reducing atmosphere to form a first carbon material, mixing the first carbon material with an inorganic compound to form an aqueous mixture, heating the aqueous mixture in an inert or reducing atmosphere to incorporate the inorganic compound into the first carbon material, removing the inorganic compound from the first carbon material to produce a second carbon material, and heating the second carbon material in an inert or reducing atmosphere to form a low oxygen content, activated carbon material.
An activated carbon material according to one embodiment can have a structural order ratio (SOR) less than or equal to 0.08, a nitrogen content greater than 0.1 wt. %, and an oxygen content less than 5 wt. %. An activated carbon material according to a further embodiment can have an oxygen content of less than 3 wt. % and a volumetric specific capacitance greater than 75 F/cm3.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description and the claims.
It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed.